a. Field of the Invention
The present invention relates generally to thrusters that provide motive power for watercraft, and, more particularly, to a thruster assembly that performs both propulsion and ballasting/dewatering functions onboard a vessel.
b. Related Art
Thrusters, as relate to waterborne vessels, are propulsive devices that are generally employed to propel and/or maneuver the vessel. As compared with shaft drives and other forms of propulsion that employ a remote power plant, thruster units commonly include an electric or hydraulic motor mounted in close association with the propeller itself in a submerged location, with electrical power or hydraulic pressure being supplied to the motor from a remote location within the hull. The propeller is frequently enclosed within a circular shroud. The motor may be reversible, and in some instances the assembly is pivotable so as to change the direction of thrust, e.g., to provide a steering effect.
Thruster units provide significant advantages in many applications, but like all propulsion systems they consume a degree of power. Power consumption is virtually always a concern in vessel design and operation, but it even more so in the case of watercraft and other vessels that are small in size and/or are intended to operate for long periods of time without refueling. Exemplary of this type of vessel are craft intended for autonomous operation such as for observation and surveillance purposes, for example. Such craft—referred to from time-to-time as unmanned autonomous vessels (UAVs)—frequently rely on wind, waves and/or sunlight as sources of energy to satisfy their power requirements in whole or in part. Typically, power requirements include not only propulsion, but steering and guidance systems, sensors onboard computing systems, and other electrical or mechanical loads as well. Moreover, some such vessels are designed for submersible operation, which necessitates pumping equipment to ballast and deballast in order to submerge and surface the craft. The low energy density of environmental sources (wind, solar, wave) means that comparatively small amounts of power can be obtained, with the result that the power budget is generally very tight. A related factor is that any added weight requires more power to propel, thus increasing energy consumption.
Much weight is the result of multiple components required to perform the above and additional functions. Furthermore, complexity and multiple components tend to both increase cost and reduce reliability, the latter again being a particularly significant consideration in the context of UAVs that must operate for extended periods with little or no human intervention. Weight and complexity also negatively impact the ability to transport, launch/retrieve and handle the craft. For example, many UAVs must be transported to distant operating areas (e.g., for military operations, ocean surveying, meteorological observations, and so on), often onboard an aircraft where weight and space are at a premium. Furthermore, after arriving at the operating area the craft must frequently be handled and launched from/recovered to a ship or other mother vessel, where excess weight can be a significant detriment. Still further, excess weight can compromise the vessel's maneuverability and responsiveness during operation.
Accordingly, there exists a need for an apparatus that enables a waterborn vessel to employ a thruster for propulsion while taking advantage of the thruster for other functions, so as to consolidate systems and reduce overall complexity and weight of the vessel. Furthermore, there exists a need for such an apparatus that can be economically constructed and that is robust and able to perform reliably without excessive maintenance.